This invention relates generally to a communication system and more specifically to a method and apparatus for determining performance margin and data rate for a Rate Adaptive Digital Subscriber Line (RADSL).
In order to make high data rate interactive services such as video conferencing and internet access available to more residential and small business customers, high speed data communication paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communications networks, and the expense of installing fiber optic cable is prohibitive. Current telephone wiring connections, which consist of copper twisted pair media, were not designed to support the high data rates required for interactive services such as video on demand or even high speed internet connections. Asymmetric Digital Subscriber Line (ADSL) technology has been developed to increase the transmission capabilities with the fixed bandwidth of existing twisted pair connections, allowing interactive services to be provided without requiring the installation of new fiber optic cable.
Discrete Multi-Tone (DMT) is a multi-carrier technique that divides the available bandwidth of a communication channel, such as a twisted pair connection, into a number of frequency sub-channels. These sub-channels are also referred to as frequency bins or carriers. The DMT technique has been adopted by the ANSI T1E1.4 (ADSL) committee for use in ADSL systems. In ADSL, DMT is used to generate 250 separate 4.3125 kHz sub-channels from 260 kHz to 1.1 MHz for downstream transmission to the end user, and 26 sub-channels from 26 kHz to 138 kHz for upstream transmission by the end user. The transmission capabilities of the individual bins are evaluated for each connection, and data is allocated to the sub-channels according to their transmission capabilities (the number of bits each bin can support). Bins that are not capable of supporting data transmission are not used, while the bit-carrying capacity of bins that can support transmission can be maximized. Thus, by using DMT in an ADSL system, the transmission capability of each twisted pair connection can be maximized over the fixed bandwidth.
Application specific requirements need to be considered in determining the transmission capability of an ADSL system. For example, given a specific channel, an increase in data rate will cause an increase in the bit error rate. For some applications a higher bit error rate may be acceptable. However, other applications may not be tolerant of an increase in bit error rate and such applications require a higher performance margin. Performance margin is a measure of a system""s immunity to an increase in noise. A system with a higher performance margin can handle a larger increase in noise on the communication channel and still maintain performance. For example, a performance margin of 6 dB indicates that a system can handle an increase of 6 dB in channel noise and still maintain its required bit error rate. The system performance of an ADSL system is dependent on its ability to accurately determine performance margins for an application specific data rate and bit error rate.
FIG. 1 illustrates a prior art ADSL system 10. ADSL system 10 comprises the remote terminal 20 coupled to the central office 30 via the twisted pair 15. Remote terminal 20 further comprises system controller 22 coupled to the remote terminal transceiver 24. Likewise, the central office 30 comprises the central office transceiver 32 coupled to the system controller 34. In the system 10 configuration illustrated, the transceivers 24, 34 would be configured to transmit at a specified data rate over the twisted pair 15. The system controllers 22 and 32 provide system control and data to the transceivers 24, 34.
In the prior art the central office 30 would transmit to the remote terminal 20 downstream options specifying data rates and configuration parameters. The downstream option would include the data rate and configuration information from the central office to the remote office. The downstream options generally comprise four data rates with configuration information for the remote terminal 20 to choose from. In the prior art, the remote terminal 20 would determine whether or not one of the four suggested data rate options met its requirements. In the event one of the data rate options was determined appropriate, the remote terminal would respond to the central office 30 specifying which data rate option was selected. In the event none of the suggested data rate options were determined appropriate, the remote terminal 20 would signal to the central office 30 that all options failed.
One limitation of the prior art is that even when one of the selected data rate options met the requirements of the remote terminal, the option may not be optimized. This occurs because the central office has blindly provided the remote terminal data rate options (blind options). The blind options provided by the central office 30 are especially troublesome when the remote terminal 20 is initiating the contact to the central office 30. When the remote terminal 20 initiates contact, the central office 30 is not aware of the requirements of the remote office. This would be the situation where the central office is capable of providing a number of different services via the ADSL system 10. Therefore, a data rate option may be accepted by the remote terminal which is not optimal.
Another suggested method of configuring an ADSL system 10 is for the central office 30 to provide initial blind options. Upon receiving the options from the central office 30, the remote terminal 20 would reject all options or select one. Where one is selected, the remote terminal would calculate a performance margin at that data rate, and return the performance margin to the central office 30. The central office, upon receiving the selected data rate and the corresponding performance margin, would then make a determination as to whether or not to transmit additional data rate suggestions to the remote terminal.
One reason for transmitting additional options is that there could be more channel capacity at a given performance margin than was allowed for in the original blind data rate options. As a result, the central office would select new options and transmit them to the remote terminal 20. In an iterative fashion, the remote terminal would select one of the options, and reply with the selected rate and a corresponding performance margin to the central office. In this manner, the central office has visibility into the remote terminal""s capabilities. A problem with this method, however, is that being an iterative process, it is not deterministic and, therefore, can take an indeterminate amount of time. Even where the process converges to a used value, the amount of time taken could require a client to wait for initialization of the system.
Yet another suggested method in the prior art used to determine data rates and configure the ADSL system, would be a method using a signal-to-noise ratio (SNR) geometric to determine a transmit rate. However, one problem with the using the SNR geometric is the that the proposed calculation of the SNR geometric is a complex calculation requiring the multiplication of the SNR values of each individual carrier, and subsequently taking the Nth root of the calculated product. Note that the Nth route corresponds to N being equal to the number of channels used. This calculation is a very time consuming calculation slowing the overall system initialization process. In addition, the multiplication of the various SNRs, which can be fractions, would result in smaller and smaller products, thereby requiring special handling where fixed point processors are used.
Therefore, a method of efficiently determining and communicating parameters used for remote terminal and central office rate option selection is needed.